The exposed portions of metal vehicle wheels can be abraded by grit and other debris from road surfaces. A hard and tough film is needed to protect the metal wheel surface. The film should be non-porous and should exhibit superior adhesion to the metal. These properties can prevent the spread of corrosion at the interface of the film and metal. However, the appearance of filiform corrosion, such as visible pitting of the coating, is not necessary for the growth of filiform corrosion. Over time, comparable film thicknesses of conventional acrylic and polyester protective clear coat powder coatings formed on aluminum wheels exhibited filiform corrosion even though the coatings appeared acceptable.
Clear coats for aluminum wheels often are applied over self-assembled monolayer (SAM) wheel surface pretreatments. Pretreating the surface with a SAM can retain the bright color and appearance of the aluminum without incorporating chrome-containing compounds. Nevertheless, clear coats applied over SAM pretreatments may still fail to provide adequate filiform corrosion resistance. Polyester clear coats applied over pre-treated aluminum wheels and trim can provide adequate filiform corrosion resistance if the clear coat is overlaid with a liquid coating to provide adequate chemical resistance, scratch resistance and weatherability.
Epoxy-containing acrylic powder compositions are also known. U.S. Pat. Nos. 5,407,707 and 5,663,240, (Simeone, et al.), disclose powder coating compositions that include epoxy functional acrylate polymers. These compositions may provide acceptable appearance when coated over a color coat but lack acceptable weatherability. German Pat. No. DE 22 61 392 A1 (Ford Werke AG) discloses powders formed from glycidyl acrylic copolymers for flexible moldings. The powders are not suitable for coatings and do not provide a weatherable molding.
U.S. Pat. App. Pub. No. 2007/0078235 A1 (Lu, et al.) discloses powder coating compositions comprising copolymers of glycidyl (meth)acrylate and caprolactone (meth)acrylate. These powder coating compositions do not exhibit filiform corrosion resistance that is better than conventional epoxy functional acrylic clear coat powder coatings.
U.S. Pat. No. 7,737,238 discloses powder coating resins comprising from 20 to 55 weight % of glycidyl acrylate (GA) or glycidyl methacrylate (GMA), preferably 25 to 50 weight %, and from 3 to 20 weight % of at least one selected from the group consisting of isobutyl methacrylate (IBMA), isobornyl acrylate (IBOA) and isobornyl methacrylate (IBOMA), preferably 5 to 15 weight %, and from 25 to 65 weight % of at least one copolymerizable ethylenically unsaturated monomer, preferably 30 to 50 weight %. These resins do not exhibit high glass transition temperatures (for example, Tg greater than 85° C.) in combination with a solubility parameter exceeding 9.15 (cal/cm3)1/2.
European Pat. No. EP 0566096 discloses the preparation and use of a cathodic electrodeposition paint that exhibit defects in the absence of a cationic acrylic copolymer. However, the copolymer adversely affects paint adhesion unless the copolymer exhibits a hydroxyl number greater than 40.
Accordingly, there is still a need for epoxy functional acrylic resins suitable for powder coating compositions that can provide improved filiform and CASS corrosion resistant powder coatings for aluminum, forged alloy or metal substrates without impairing powder coating smoothness, clarity or powder coating composition stability and impact resistance, especially in powder coatings for aluminum or forged alloy wheels and automotive trim pretreated with, for example, hexavalent chromium-free pretreatments.